The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

MH370 search crews 'will not rest': Abbott

Authorities can't be certain of success in the search for flight MH370, but won't rest until they have done everything possible to find the aircraft, Prime Minister Tony Abbott says.

Mr Abbott met with Malaysian Prime Minister Najib Razak in Perth on Thursday, saying it had been a tough time for him as search crews scour the ocean more than 1600 kilometres off the West Australian coast.

"It is a very difficult search - the most difficult in human history - but as far as Australia is concerned, we are throwing everything we have at it," Mr Abbott said.

He said the search area, which had moved north, was constantly being refined, but was still a remote and inaccessible location with difficult sea conditions.

"We cannot be certain of ultimate success in the search for MH370, but we can be certain that we will spare no effort, that we will not rest until we have done everything we humanly can," he said.

"This is an example of how the countries of the world can work together. This is one of those times when we are all citizens of one world and we are all global citizens."

Mr Abbott said Australia owed it to the 239 people on board the Malaysia Airlines flight and the "troubled citizens of the wider world" to solve the extraordinary mystery.

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"I know this is an extraordinarily difficult, indeed devastating time for you, but we will not let you down," he said.

Mr Najib said the commitment of search crews would not be forgotten, and any differences between the 26 nations involved in the search had been set aside..

"They have searched through stormy seas and freezing fog ... we owe them each a debt of gratitude," he said.

"The disappearance of MH370 has tested our collective resolve.

"I believe the courage of the crews is more than equal to the task."

Mr Abbott said it was "typical of the grace of the man" that Mr Najib came to Australia and to RAAF base Pearce to personally thank those involved in the search.

The initial search focused on the South China Sea. After subsequent searches in the Strait of Malacca, Andaman Sea and northern Indian Ocean, the focus turned to the southern Indian Ocean, based on satellite data.

Mr Najib said he knew many families could not grieve until the plane was found and promised search crews would not give up.

Retired air chief marshal Angus Houston, who is leading the new Joint Agency Coordination Centre for the search, said it was one of the most complex searches in the world and an example of international co-operation at its best.

The flight disappeared on March 8 while flying from Kuala Lumpur to Beijing.

Hundreds of objects have been spotted in the search area, but so far none have been verified as the plane's wreckage.

Eight planes and nine ships searched an area of 223,000 square kilometres, 1680 kilometres west-northwest of Perth on Thursday.